Herpes simplex virus is a useful model for studying the mechanisms involved in DNA cells. Our current efforts are directed toward studying this process with purified proteins. Seven viral genes are both necessary and sufficient to carry out authentic origin-dependent DNA replication. Ongoing biochemical analyses in several laboratories support the idea that the products of these seven genes all participate directly in viral DNA synthesis. We are currently using both biochemical and molecular genetic approaches to understand the function of these polypeptides in detail. The HSV DNA polymerase purified from infected HeLa cells consists of a stable complex of at least two polypeptides: UL30, the catalytic subunit, and UL42. We have purified both subunits free of each other using recombinant baculoviruses. Although the purified catalytic subunit has the same specific activity as the UL30/UL42 complex using activated DNA as template, the complex is much more active on a long single-stranded DNA template primed with a short oligonucleotide. Several lines of evidence support the view UL42 increases the efficiency of the DNA polymerase by increasing its processivity. The UL9 protein binds specifically to the HSV origins of replication. Determination of the hydrodynamic properties of purified UL9 protein has shown that the protein interacts with itself to form a dimer in solution, and possibly higher order structures when bound to the origin of replication. Current experiments on UL9 are aimed at characterizing these higher order structures in more detail. In addition to origin binding, the purified UL9 protein an intrinsic 3' to 5' helicase activity that is capable of unwinding short duplex DNA segments in an ATP-dependent reaction. The role of this unwinding reaction in DNA replication is being investigated. The UL5, UL8, and UL52 polypeptides form a three protein complex that has both helicase and primase activities. Data from several labs suggest that UL8 has primase activity, and UL5 and UL52 act together as a helicase. We are currently carrying out directed mutagenesis of these proteins to gain further insight into their activities.